motion tunes gain value of a webcam despite fixed value in video_params parameter

[alexejpissarev]

I use motion on a raspberry pi with a connected webcam in a remote test lab.
Sometimes there is a lot of light there when the "motion-pi" is starting, sometimes it boots in the dark.

I've observed that the gain value of v4l2 driver was set to max (v4l2-ctl --device=/dev/video --list-ctrls --> gain 0x00980913 (int) : min=0 max=255 step=1 default=0 value=255) if the pi had started in the dark.

That is why I tried to set fixed gain value via video_params parameter, e.g.
video_params "Gain"=12

I expected to see this value in the output of v4l2-ctl after reboot, no matter of light conditions.
But motions seems to still tune it by the first start:

1. initially set gain value to e.g. 13, just to know the starting value (v4l2-ctl --device=/dev/video -c gain=13)
2. set video_params in the motion config to "Gain"=12
3. switch off all lights in the lab
4. reboot the "motion pi"
5. check v4l2-ctl --device=/dev/video --list-ctrls --> gain value is set to 255 (max)
6. switch on lights
7. reboot the "motion pi"
8. check v4l2-ctl --device=/dev/video --list-ctrls --> gain value is set to something like 100

Strangely motion do set gain to value defined via video_params if motion service is just restarted afterwards:
9. systemctl restart motion
10. check v4l2-ctl --device=/dev/video --list-ctrls --> gain value is set to 12

Is there a possibility to prevent tuning the gain value in case this value is defined in the motion configuration, in the video_params parameter?

Thank you in advance for the clarification!

Kind regards,
Alexej

Here some details:
motion version: 4.7.0
installed as a deb package
standalone or part of third party: motion
video stream source: usb webcam "Logitech Webcam C930e"
hardware: ARM RPi4
operating system: Debian GNU/Linux 12 (bookworm)

[alexejpissarev]

as it turned out the "problem" was the v4l2 library itself: v4l2 seems to tune some webcam controls during every reboot and to set e.g. gain according to light conditions etc.
Even if I tried to set gain to e.g.5 via v4l2-cli, it jumped to, say, 127 by first try and only by the second one gain=5 has been accepted.
I guess the same happens when motion tries to set this control as defined in the motion config - v4l2 make then its own "decision".